1. Field of the Invention
The present invention relates to a zoom lens and an image pick-up apparatus equipped with the zoom lens; the zoom lens may be suitable for image pick-up optical systems, such as single lens reflex (SLR) cameras, digital still cameras, digital video cameras, TV cameras, and surveillance cameras.
2. Description of the Related Art
Known focusing methods for zoom lenses include front-lens focusing, in which a first lens unit on an object side is moved, and inner focusing and rear focusing, in which the second and subsequent lens units are moved.
Conventionally, inner-focus and rear-focus zoom lenses can be easily reduced in size because the first lens unit in these types of zoom lens typically has a smaller effective diameter than the first lens unit of front-lens focusing zoom lenses. In addition, because relatively small and light-weight lens units are moved to achieve focus, quick focusing can be easily performed particularly in cameras having an auto-focus function.
An example of zoom lenses that can easily achieve a high zoom ratio is a positive-lead type zoom lens, in which the lens unit on the extreme object side is a lens unit having a positive refractive power. An example of such positive-lead type zoom lenses is a zoom lens that includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a rear lens group including a plurality of lens units and having a positive refractive power as a whole and that performs zooming by changing the distances between the lens units.
Such positive-lead type zoom lenses include a zoom lens that performs focusing by moving the second lens unit having a negative refractive power.
In positive-lead type zoom lenses, the second lens unit having a negative refractive power mainly changes the magnification. The second lens unit having a negative refractive power has a negative magnification factor at the wide-angle end, and the absolute value of the magnification increases with zooming toward the telephoto end (in the direction from the negative magnification factor toward −1). This tendency becomes clearer, particularly in the case of zoom lenses having a high zoom ratio.
The image-forming magnification of a focus lens unit and the focus sensitivity (the ratio between the distance of movement of the focus lens unit per unit and the amount of shift of focus) are expressed by the following expression:ES=(1−βf2)×βr2 where ES is the focus sensitivity, βf is the image-forming magnification of the focus lens unit, and βr is the combined magnification of all the lens units disposed on the image side with respect to the focus lens unit.
The above expression shows that the focus sensitivity, which is 0 when the absolute value of the image-forming magnification, βf, of the focus lens unit is 1, increases as the absolute value of the image-forming magnification gets farther from 1.
In many positive-lead type zoom lenses, the second lens unit having a negative refractive power changes the magnification in the direction from the negative magnification factor toward −1, during zooming from the wide-angle end to the telephoto end. Thus, when an image of a near object is captured using a zoom lens having a high zoom ratio, the focus sensitivity of the second lens unit is low near the telephoto end, and the distance of movement of the focus lens unit increases. Furthermore, when the image-forming magnification of the second lens unit reaches −1 while changing magnification, the focus sensitivity becomes 0; this disables focusing.
Therefore, when a high zoom ratio is to be achieved with a positive-lead type zoom lens employing a rear focusing method, it is important to arrange the refractive powers of the lens units and select the focusing lens unit appropriately so that the focus sensitivity does not become too small. Setting of the refractive powers of the lens units located on the object side with respect to the focusing lens unit is particularly important. If inappropriate refractive powers are set, it is difficult to achieve focus over a wide object distance while suppressing an increase in distance of movement of the focusing lens unit.
The present invention provides a zoom lens having a high zoom ratio, which can achieve focus without significantly increasing the distance of movement of the focus lens unit, and an image pick-up apparatus having the zoom lens.